Make force_ro consistent with other sysfs entries.

Fixes: 371a689f ('mmc: MMC boot partitions support')
Cc: Andrei Warkentin <andrey.warkentin@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>